Breakjunction for molecular contacting

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Preparation of breakjunction for molecular contacting based on electromigration.

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Breakjunction for molecular contacting

  1. 1. François Bianco, July 10, 2007 Break junction - p. 1/35 Break junction for molecular electronics using electromigration François Bianco
  2. 2. Introduction q Outline q Idea and History q Realization Electromigration process Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 2/35 Introduction
  3. 3. Introduction q Outline q Idea and History q Realization Electromigration process Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 3/35 Outline 1. Long terms goal and interest of molecular electronics 2. Electromigration process 3. Breaking phases 4. Quantization of the conductance 5. Experimental setup 6. Results s Conductance quantization s Electromigration process s Gap size s Critical power s Feedback algorithm 7. Conclusion
  4. 4. Introduction q Outline q Idea and History q Realization Electromigration process Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 4/35 Idea and History Idea s Use molecule as building block for passive and active electronic components s Extend the Moore’s law beyond the foreseen limit of common silicon electronics s Access to new quantum effects History s 1940 first theoretical explanation of charge transfer in molecules s 1988 theoretical single-molecule field-effect transistor s 1997 first measurement of a single molecule conductance
  5. 5. Introduction q Outline q Idea and History q Realization Electromigration process Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 5/35 Realization The first problem arising is the fabrication of molecular-scale electrical contacts. The use of: s Scanning tunneling microscope manipulation s Atomic force microscope manipulation s Mechanical break junction s or Electromigration break junction allows one to reach nanometer-spaced electrodes.
  6. 6. Introduction Electromigration process q History q Description q Forces q Diffusion q Activation energy q Joule Heating Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 6/35 Electromigration process
  7. 7. Introduction Electromigration process q History q Description q Forces q Diffusion q Activation energy q Joule Heating Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 7/35 History s Failure mechanism of small wires and electronics s Discovered more than 100 years ago by Gerardin a French scientist s Became practical only in the 60s for electronics design s 1968 James R. Black wrote his famous equation describing the mean time before failure due to electromigration3 (CC-BY-SA) Patrick-Emil Zörner
  8. 8. Introduction Electromigration process q History q Description q Forces q Diffusion q Activation energy q Joule Heating Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 8/35 Description Electromigration (EM) is the ion mass flux driven by a high electrical current density. s Due to collisions between the moving electrons and the ions s Two types of failure: x Open circuit x Short circuit
  9. 9. Introduction Electromigration process q History q Description q Forces q Diffusion q Activation energy q Joule Heating Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 9/35 Forces There is two forces acting on the ions s Electrostatics force due to the applied voltage Fe s Electron wind due to the momentum transfer from the electrons to the ions Fp F = Fe − Fp. = Z∗ eE (1)
  10. 10. Introduction Electromigration process q History q Description q Forces q Diffusion q Activation energy q Joule Heating Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 10/35 Diffusion The migration of ions takes place where the symmetry is broken like at: s the grain boundaries s the surface s or within the lattice at high temperature
  11. 11. Introduction Electromigration process q History q Description q Forces q Diffusion q Activation energy q Joule Heating Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 11/35 Activation energy Only the activated ions could participate to the diffusion, this is reflected by the temperature dependent diffusion coefficient: D = D0e −EA kT (2) where EA is the activation energy.
  12. 12. Introduction Electromigration process q History q Description q Forces q Diffusion q Activation energy q Joule Heating Conductance quantization Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 12/35 Joule Heating The power dissipated in the junction P∗ = v∗2 Rj . (3) increase the local temperature accelerating the process by feedback mechanisms.
  13. 13. Introduction Electromigration process Conductance quantization q Quantization (1) q Quantization (2) q Breaking phases Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 13/35 Conductance quantization
  14. 14. Introduction Electromigration process Conductance quantization q Quantization (1) q Quantization (2) q Breaking phases Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 14/35 Quantization (1) To get a theoretical explanation of the quantization use the following steps 1. Solve the Schrödinger’s equation 2. Assumption translation symmetry in y direction 3. Separate the wavefunction 4. Plug the wavefunction into the current density Contribution to the current density of the electron in mode nky jnky = −e 1 L |χn(x, z)|2 ρ ky m∗ ey v (4) ρ charge carrier density, v the group velocity
  15. 15. Introduction Electromigration process Conductance quantization q Quantization (1) q Quantization (2) q Breaking phases Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 15/35 Quantization (2) The total current is the sum over ky and n. s Cross section determines the boundaries conditions s Use the Pauli Exclusion Principle s Possible n bellow the Fermi energy in the wire s Conductance shows plateaus at integer multiples of the conductance quantum G0 = 2e2 h .
  16. 16. Introduction Electromigration process Conductance quantization q Quantization (1) q Quantization (2) q Breaking phases Experimental setup Results Conclusion François Bianco, July 10, 2007 Break junction - p. 16/35 Breaking phases 1. Bulk regime -> continuous resistance (diffusive regime) 2. Intermediate steps 3. QPC -> discrete resistance due to the size reduction
  17. 17. Introduction Electromigration process Conductance quantization Experimental setup q Fabrication q Geometry and sizes q Four point measurement q Feedback algorithm Results Conclusion François Bianco, July 10, 2007 Break junction - p. 17/35 Experimental setup
  18. 18. Introduction Electromigration process Conductance quantization Experimental setup q Fabrication q Geometry and sizes q Four point measurement q Feedback algorithm Results Conclusion François Bianco, July 10, 2007 Break junction - p. 18/35 Fabrication s The junctions (d) –> EBM s The connection pads –> Photolithography
  19. 19. Introduction Electromigration process Conductance quantization Experimental setup q Fabrication q Geometry and sizes q Four point measurement q Feedback algorithm Results Conclusion François Bianco, July 10, 2007 Break junction - p. 19/35 Geometry and sizes The connectors are designed for minimizing the resistance for the voltage pads. We build two junctions with two different geometries: s Wire s Bowtie Device Geometry Length (nm) Width (nm) Thickness (nm) 1 & 2 wire 500 70 30 3 & 4 wire 500 75 30 7 & 8 wire 400 80 30 02 bowtie - 100 30
  20. 20. Introduction Electromigration process Conductance quantization Experimental setup q Fabrication q Geometry and sizes q Four point measurement q Feedback algorithm Results Conclusion François Bianco, July 10, 2007 Break junction - p. 20/35 Four point measurement Objective: reduce the error for the resistance measurement
  21. 21. Introduction Electromigration process Conductance quantization Experimental setup q Fabrication q Geometry and sizes q Four point measurement q Feedback algorithm Results Conclusion François Bianco, July 10, 2007 Break junction - p. 21/35 Feedback algorithm Goals: s EM in a controlled fashion s to reach the latest conductance plateau s and avoid a runaway of the EM Implementation: s Apply voltage ramps s Control the evolution with different feedback mechanisms x Resistance dR x Normalized conductance G/G0 x Normalized breaking rate 1 R ∂R ∂t
  22. 22. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 22/35 Results
  23. 23. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 23/35 Electromigration A->B : ohmic response B->C : controlled breaking C : break point
  24. 24. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 24/35 Quantization Instabilities: fluctuation between allowed atomic arrangements
  25. 25. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 25/35 Statistical occurence 8 measurements were added Non-integer value due to: s To small number of measured junction8 s Occurrence of non-integer value in Gold?
  26. 26. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 26/35 Gaps sizes The sizes were approximated from SEM pictures. Size Number Yield < 10 nm 7 23% 10 − 20 nm 5 17% 20 − 50 nm 1 7% Low yields: Feedback take too long to detect the break point (0.1 to 1 s) Reorganization of the atoms the surface
  27. 27. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 27/35 Gaps sizes SEM (1)
  28. 28. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 28/35 Gaps sizes SEM (2)
  29. 29. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 29/35 Critical power (1) v∗ = P∗ G(1 − GRs) (5) s Rs approximated as the start resistance s fitting parameter P∗ s use least-square algorithm
  30. 30. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 30/35 Critical power (2) Geometry Mean critical power (µW) Standard deviation (µW) Wire 147 27 Bowtie 158 42 s Feedback mechanism not adapted s No good approximation for the series resistance s Wrong idea for the fitting
  31. 31. Introduction Electromigration process Conductance quantization Experimental setup Results q Electromigration q Quantization q Statistical occurence q Gaps sizes q Gaps sizes SEM (1) q Gaps sizes SEM (2) q Critical power (1) q Critical power (2) q Power feedback Conclusion François Bianco, July 10, 2007 Break junction - p. 31/35 Power feedback Feedback do not step down the voltage at a constant value : Conclusion: Feedback only prevents a runaway EM but is not able to detect it.
  32. 32. Introduction Electromigration process Conductance quantization Experimental setup Results Conclusion q Summary q References q Questions ? François Bianco, July 10, 2007 Break junction - p. 32/35 Conclusion
  33. 33. Introduction Electromigration process Conductance quantization Experimental setup Results Conclusion q Summary q References q Questions ? François Bianco, July 10, 2007 Break junction - p. 33/35 Summary Break junction s EM process observed s Quantization of conductance seen Feedback s Need to detect sooner the break point s No able to detect the EM but avoid a runaway of the process
  34. 34. Introduction Electromigration process Conductance quantization Experimental setup Results Conclusion q Summary q References q Questions ? François Bianco, July 10, 2007 Break junction - p. 34/35 References 1. Wikipedia 2. (CC-BY-SA) Patrick-Emil Zörner
  35. 35. Introduction Electromigration process Conductance quantization Experimental setup Results Conclusion q Summary q References q Questions ? François Bianco, July 10, 2007 Break junction - p. 35/35 Questions ? Science has explained nothing; the more we know the more fantastic the world becomes and the profounder the surrounding darkness. [Aldous Leonard Huxley] The important thing is not to stop questioning. [Albert Einstein]

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